In magnetic field based non-contact current sensors, current through a low-resistance metal trace creates a magnetic field that is sensed by a magnetic sensor. Intensity of the created magnetic field is proportional to the current flowing through the trace. A differential sensor placement is typically used to cancel any uniform external background magnetic field as common-mode signal. However, a gradient in the external magnetic field can still create significant differential signal interference, which limits the accuracy in measurement. A gradient in the magnetic field can be produced by the proximity of a magnet, other current carrying trace, or even magnetic metal to the current sensor, which changes Earth's relatively uniform magnetic field in the local vicinity.
An example of a current carrying trace causing interference is shown in FIG. 5, which shows system 500 containing two magnetic field based current sensors 504A and 504B. In system 500, the current in trace 502 is to be measured. As trace 502 nears the sensor location, trace 502 is narrowed to concentrate the magnetic field produced by the current and the current is directed into a current loop that includes two parallel sections 502A and 502B and curved section 502C, such that the current in sections 502A and 502B is flowing in opposite directions. Magnetic field based current sensor 504A measures the current in trace section 502A and magnetic field based current sensor 504B measures the current in trace section 502B.
Trace 508 carries a current that can interfere with the measurements of trace 502. The distance D1 between trace 508 and trace section 502A is 7.5 mm while the distance D2 between trace sections 502A and 502B is 6.4 mm. Trace 508 is also offset from trace 502 by 1.48 mm in the Z-axis. Table 1 below shows the difference in interference between magnetic field based current sensors 504A and 504B.
TABLE 1ParametersMeasured DataCurrent Sensitivity (Differential)168mV/ACurrent resolution (Bandwidth Normalized)10μA/√HzCurrent resolution (Bandwidth: 10 kHz)1mAInterference on Left sensor12mV/AInterference on Right sensor6mV/ADifferential interference6mV/AEquivalent current interference35.7mA/A
Prior solutions to the problem of interference have included:
A. Reducing the distance between the sensors;
B. Carefully routing other current trace(s); and
C. Magnetic shielding using high permeability material.
However, the minimum distance between sensors in solution A is limited by the design, while remnant interference can still be significant. The routing solutions are only applicable for interference from current traces and do not address external sources such as a magnet. Solution B also limits the routing options, while solution C is inherently expensive.